RE: [PATCH v3 20/35] x86/bugs: Define attack vectors

Kaplan, David
2025-02-12T17:21:03Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Josh Poimboeuf <jpoimboe@kernel.org>
> Sent: Tuesday, February 11, 2025 12:08 PM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>; Borislav Petkov <bp@alien8.de>; Peter
> Zijlstra <peterz@infradead.org>; Pawan Gupta
> <pawan.kumar.gupta@linux.intel.com>; Ingo Molnar <mingo@redhat.com>; Dave
> Hansen <dave.hansen@linux.intel.com>; x86@kernel.org; H . Peter Anvin
> <hpa@zytor.com>; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v3 20/35] x86/bugs: Define attack vectors
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Wed, Jan 08, 2025 at 02:25:00PM -0600, David Kaplan wrote:
> > Define 5 new attack vectors that are used for controlling CPU
> > speculation mitigations and associated command line options. Each
> > attack vector may be enabled or disabled, which affects the CPU
> > mitigations enabled.
> >
> > The default settings for these attack vectors are consistent with
> > existing kernel defaults, other than the automatic disabling of VM-based
> > attack vectors if KVM support is not present.
> >
> > Signed-off-by: David Kaplan <david.kaplan@amd.com>
> > ---
> > arch/x86/include/asm/bugs.h | 11 +++++++
> > arch/x86/kernel/cpu/bugs.c | 60
> +++++++++++++++++++++++++++++++++++++
> > 2 files changed, 71 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/bugs.h b/arch/x86/include/asm/bugs.h
> > index f25ca2d709d4..354d04a964f0 100644
> > --- a/arch/x86/include/asm/bugs.h
> > +++ b/arch/x86/include/asm/bugs.h
> > @@ -12,4 +12,15 @@ static inline int ppro_with_ram_bug(void) { return 0; }
> >
> > extern void cpu_bugs_smt_update(void);
> >
> > +enum cpu_attack_vectors {
> > + CPU_MITIGATE_USER_KERNEL,
> > + CPU_MITIGATE_USER_USER,
> > + CPU_MITIGATE_GUEST_HOST,
> > + CPU_MITIGATE_GUEST_GUEST,
> > + CPU_MITIGATE_CROSS_THREAD,
> > + NR_CPU_ATTACK_VECTORS,
> > +};
> > +
> > +bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v);
> > +
> > #endif /* _ASM_X86_BUGS_H */
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index aee2945bdef9..88eba8e4c7fb 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -169,6 +169,66 @@
> DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
> > DEFINE_STATIC_KEY_FALSE(mmio_stale_data_clear);
> > EXPORT_SYMBOL_GPL(mmio_stale_data_clear);
> >
> > +#ifdef CONFIG_CPU_MITIGATIONS
> > +/*
> > + * All except the cross-thread attack vector are mitigated by default.
> > + * Cross-thread mitigation often requires disabling SMT which is too expensive
> > + * to be enabled by default.
> > + *
> > + * Guest-to-Host and Guest-to-Guest vectors are only needed if KVM support is
> > + * present.
> > + */
> > +static bool cpu_mitigate_attack_vectors[NR_CPU_ATTACK_VECTORS]
> __ro_after_init = {
> > + [CPU_MITIGATE_USER_KERNEL] = true,
> > + [CPU_MITIGATE_USER_USER] = true,
> > + [CPU_MITIGATE_GUEST_HOST] = IS_ENABLED(CONFIG_KVM),
> > + [CPU_MITIGATE_GUEST_GUEST] = IS_ENABLED(CONFIG_KVM),
> > + [CPU_MITIGATE_CROSS_THREAD] = false
> > +};
> > +
> > +#define DEFINE_ATTACK_VECTOR(opt, v) \
>
> s/opt/name/ to distinguish it from v.
>
> > + static int __init v##_parse_cmdline(char *arg) \
>
> Instead of "CPU_MITIGATE_USER_KERNEL_parse_cmdline" it should really be
> "mitigate_user_kernel_cmdline".
>
> Also this line shouldn't be indented.
>
> Also it's more readable to tab align all the line continuation
> backslashes.
>
> > +{ \
> > + if (!strcmp(arg, "off")) \
> > + cpu_mitigate_attack_vectors[v] = false; \
> > + else if (!strcmp(arg, "on")) \
> > + cpu_mitigate_attack_vectors[v] = true; \
> > + else \
> > + pr_warn("Unsupported " opt "=%s\n", arg); \
> > + return 0; \
> > +} \
> > +early_param(opt, v##_parse_cmdline)
> > +
> > +bool cpu_mitigate_attack_vector(enum cpu_attack_vectors v)
> > +{
> > + if (v < NR_CPU_ATTACK_VECTORS)
> > + return cpu_mitigate_attack_vectors[v];
> > +
> > + WARN_ON_ONCE(v >= NR_CPU_ATTACK_VECTORS);
> > + return false;
> > +}
>
> This error can be checked at build time.
>
> > +#else
>
> This needs a /* !CONFIG_CPU_MITIGATIONS */ comment.
>
> > #endif
>
> As does this.
>
>
> So, something like so:
>
> #ifdef CONFIG_CPU_MITIGATIONS
> /*
> * All except the cross-thread attack vector are mitigated by default.
> * Cross-thread mitigation often requires disabling SMT which is too expensive
> * to be enabled by default.
> *
> * Guest-to-Host and Guest-to-Guest vectors are only needed if KVM support is
> * present.
> */
> static bool cpu_mitigate_attack_vectors[NR_CPU_ATTACK_VECTORS]
> __ro_after_init = {
> [CPU_MITIGATE_USER_KERNEL] = true,
> [CPU_MITIGATE_USER_USER] = true,
> [CPU_MITIGATE_GUEST_HOST] = IS_ENABLED(CONFIG_KVM),
> [CPU_MITIGATE_GUEST_GUEST] = IS_ENABLED(CONFIG_KVM),
> [CPU_MITIGATE_CROSS_THREAD] = false
> };
>
> #define DEFINE_ATTACK_VECTOR(name, v) \
> static int __init name##_parse_cmdline(char *arg) \
> { \
> if (!strcmp(arg, "off")) \
> cpu_mitigate_attack_vectors[v] = false; \
> else if (!strcmp(arg, "on")) \
> cpu_mitigate_attack_vectors[v] = true; \
> else \
> pr_warn("Unsupported " __stringify(name) "=%s\n", arg); \
> return 0; \
> } \
> early_param(__stringify(name), name##_parse_cmdline)
>
> #define cpu_mitigate_attack_vector(v) \
> ({ \
> BUILD_BUG_ON(v >= NR_CPU_ATTACK_VECTORS); \
> cpu_mitigate_attack_vectors[v]; \
> })
>
> #else /* !CONFIG_CPU_MITIGATIONS */
>
> #define DEFINE_ATTACK_VECTOR(name, v) \
> static int __init name##_parse_cmdline(char *arg) \
> { \
> pr_crit("Kernel compiled without mitigations, ignoring %s; system may still be
> vulnerable\n", \
> __stringify(name)); \
> return 0; \
> } \
> early_param(__stringify(name), name##_parse_cmdline)
>
> #define cpu_mitigate_attack_vector(v) false
>
> #endif /* !CONFIG_CPU_MITIGATIONS */
>
> DEFINE_ATTACK_VECTOR(mitigate_user_kernel,
> CPU_MITIGATE_USER_KERNEL);
> DEFINE_ATTACK_VECTOR(mitigate_user_user,
> CPU_MITIGATE_USER_USER);
> DEFINE_ATTACK_VECTOR(mitigate_guest_host,
> CPU_MITIGATE_GUEST_HOST);
> DEFINE_ATTACK_VECTOR(mitigate_guest_guest,
> CPU_MITIGATE_GUEST_GUEST);
> DEFINE_ATTACK_VECTOR(mitigate_cross_thread,
> CPU_MITIGATE_CROSS_THREAD);

Got it, thanks

--David Kaplan